Old Homart wont fire by thermostat

I have an ollld house I want to sell and it has an ollld Sears Homart furnace [#292 13015]. I can only get the firebox to start by manually moving the lever in lower right side of furnace cavity. It will run until heated to my Thermostat setting but will not kick back on when the house &quot;colds down&quot;. Could this be thermocouple; my thermostat or whatever? The thermostat did work with my AC unit this summer. thanks